The invention relates generally to charging of rechargeable energy storage systems (e.g., batteries and electric double-layer capacitors and the like) and more particularly to charging storage systems in conditions when a peak line voltage is greater than a storage system voltage, such as may be the case, for example, with a low state-of-charge (SOC) for the energy storage system.
The voltage across the energy storage system varies as a function of SOC and line voltages used for recharging have an inherent variation as well. Relevant to this invention is the difference between the peak line voltage available to the charger and the voltage across the energy storage device being charged. In many high-energy applications (e.g., electric vehicles and industrial applications and the like), the energy storage system includes parallel and series connected cells and modules that have a net voltage greater than typical line voltages. In these typical cases, the charging systems include a voltage converter to up-convert, or boost, the line voltage to the desired level for charging.
Rechargeable energy storage systems are often charged from an AC line source that includes characteristics of a peak voltage (V) and a volts root mean square (VRMS). For a sinusoidal voltage, average voltage VRMS is equal to peak voltage divided by the square root of two. In the United States, conventional line voltages are identified as average voltages and are supplied at about 110 VRMS though sometimes 240 VRMS is supplied (and is often single phase though three-phase is not uncommon). Thus, 110 VRMS is about equal to 155 Volts (peak). Of course, other countries may use other values.
For an example, consider use of 240 VRMS as a nominal line voltage. The actual line voltage varies over time, for this example it will be a +/−10% voltage variation (e.g., a +10% greater voltage for 240 VRMS means the peak voltage could be about 370 Volts (240 VRMS*1.1*1.414)). The 370 Volts is compared to the voltage across the energy storage system. In some cases, when the energy storage system is at a lower end of its SOC, the voltage across it system may be ˜330 Volts which is less than the maximum peak line voltage of 370 Volts.
The topology of many voltage converters is often such that the voltage converter cannot charge the energy storage system in this mode, that is when the peak line voltage exceeds the voltage of the component being charged. (Depending upon circuit topology, one reason the voltage converter cannot charge battery assembly is because the described relative voltages results in uncontrollable current flow out of the converter, potentially seriously damaging the converter and/or the energy storage system.) This is undesirable to say the least, particularly given the consequences of damage and costs of the components in an electric vehicle or other high-energy system.
Solutions for electric vehicles must often meet tight budgets for space, weight, and component costs. Solutions to address the special case of a too-high line voltage relative to the voltage of the energy storage system advantageously have low impact on the existing design in terms of additional components, circuits, and the like.
What is needed is a charging system that is capable of providing high energy to a high performance energy storage assembly for charging the energy storage assembly while efficiently and safely handling conditions of a too “high” line-in voltage relative to a voltage level of the energy storage assembly.